With regard to Gardner's Grendel, I struggle with the sense that Grendel gives "to 'his' people the kinds of 'boons' or benefits that an epic hero gives." I can't find who Grendel's people are (other monsters?)—he is portrayed as a loner. One that is mentioned is his mother—simple-minded, with no language. Grendel talks with the dragon, but he is not of Grendel's race.
So I look to the characteristics of the "epic hero." Grendel comes from legend...in this he fits the description. Grendel does not descend from a deity, but from Cain (the murder of Abel). Grendel (according to the Shaper) is the product of the feud of "famous" brothers—and the descendant of Cain—who is presented as a "father" to a race of demons:
He told of an ancient feud between two brothers which split all the world between darkness and light. And I, Grendel, was the dark side...The terrible race God cursed.
However, as the story is told, Cain is almost mythical in terms of being able to create such a race, so maybe this detail fits if we choose to stretch the meaning of "deity."
The definition of an epic hero also states...
The hero...faces adversaries that try to defeat him...
This is also the case with Grendel: his adversaries are the Danes, and are inflamed by their fear of him, especially in light of the Shaper's condemnation of the creature in his stories. When Grendel grovels before the Danes, asking for mercy, they attack and drive him away.
While Grendel is "larger than life," he does nothing that seems typically heroic: there are no great accomplishments. But he does show that he is a creature of almost unbeatable strength, he is a warrior, and he...
... performs extraordinary tasks that most find difficult.
According to the dragon, the "extraordinary task" Grendel performs is making mankind more human:
Ah, Grendel!...You improve them, my boy! Can't you see that yourself? You stimulate them! You make them think and scheme. You drive them to poetry, science, religion, all that makes them what they are for as long as they last. You are, so to speak, the brute existent by which they learn to define themselves. The exile, captivity, death they shrink from—the blunt facts of their mortality, their abandonment—that's what you make them recognize, embrace! You are mankind or man's condition: inseparable as the mountain-climber and the mountain.
So here Grendel is giving to mankind (but not to his own).
The epic hero also has "human traits." Grendel has...
"a sense of humor and a gift for language." Grendel even has a weakness for poetry...Grendel strives, however comically, to escape from his baseness.
Grendel does, in fact, exhibit human traits. He has humor, language and likes poetry. He strives to be better than he is. So paradoxically, how much less human is he than Unferth, who murdered his brothers? In Chapter Seven, Wealtheow stops to serve Unferth mead (a honey wine). He refuses it; the Queen wonders why until one "made bold by mead" says...
Men have been known to kill their brothers when they've too much mead. Har, har.
Grendel is openly murderous, but Unferth hides his evil behind a mask of heroism.
I agree with your observance that even as a monster, Grendel offers the Danes the benefit of an enemy that unites them. With these details in mind, it can be argued that Grendel might be seen, in some basic ways, as an epic hero.
(Ultimately, his degradation into a blood-thirty killer makes it hard to uphold this argument.)